Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are expected to become an increasingly popular form of flat panel display technology. This is because OLEDs have a wide variety of potential applications, including cell phones, personal digital assistants (PDAs), computer displays, informational displays in vehicles, television monitors, as well as light sources for general illumination. Due to their bright colors, wide viewing angle, compatibility with full motion video, broad temperature ranges, thin and conformable form factor, low power requirements and the potential for low cost manufacturing processes, OLEDs are seen as a future replacement technology for cathode ray tubes (CRTs) and liquid crystal displays (LCDs), which currently dominate the growing $40 billion annual electronic display market. Due to their high luminous efficiencies, electrophosphorescent OLEDs are seen as having the potential to replace incandescent, and perhaps even fluorescent, lamps for certain types of applications.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers between an anode and a cathode. The material of one of these layers is specifically chosen based on the material's ability to transport holes, a “hole transporting layer” (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an “electron transporting layer” (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the HTL, while the cathode injects electrons into the ETL. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone, while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest HOMO-LUMO energy gap. Recombination of the short-lived excitons may be visualized as an electron dropping from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO), with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism.
The materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electroluminescent emission occur. Such OLEDs are referred to as having a “single heterostructure” (SH). Alternatively, the electroluminescent material may be present in a separate emissive layer between the HTL and the ETL in what is referred to as a “double heterostructure” (DH).
In a single heterostructure OLED, either holes are injected from the HTL into the ETL where they combine with electrons to form excitons, or electrons are injected from the ETL into the HTL where they combine with holes to form excitons. Because excitons are trapped in the material having the lowest energy gap, and commonly used ETL materials generally have smaller energy gaps than commonly used HTL materials, the emissive layer of a single heterostructure device is typically the ETL. In such an OLED, the materials used for the ETL and HTL should be chosen such that holes can be injected efficiently from the HTL into the ETL. Also, the best OLEDs are believed to have good energy level alignment between the HOMO levels of the HTL and ETL materials.
In a double heterostructure OLED, holes are injected from the HTL and electrons are injected from the ETL into the separate emissive layer, where the holes and electrons combine to form excitons.
Light emission from OLEDs has typically been via fluorescence, however OLED emission via phosphorescence has been recently demonstrated. As used herein, the term “phosphorescence” refers to emission from a triplet excited state of an organic molecule and the term “fluorescence” refers to emission from a singlet excited state of an organic molecule. The term luminescence refers to either fluorescent or phosphorescent emission.
Successful utilization of phosphorescence holds enormous promise for organic electroluminescent devices. For example, an advantage of phosphorescence is that potentially all excitons formed by the recombination of holes and electrons, either as a singlet or triplet excited state, may participate in luminescence. This is because the lowest singlet excited a state of an organic molecule is typically at a slightly higher energy than the lowest triplet excited state. For example, in typical phosphorescent organometallic compounds, the lowest singlet excited state may rapidly decay to the lowest triplet excited state, from which the phosphorescence is produced. In contrast, only a small percentage (about 25%) of excitons in fluorescent devices are capable of producing the fluorescent luminescence that is obtained from a singlet excited state. The remaining excitons in a fluorescent device, which are produced in the lowest triplet excited state, are typically not capable of being converted into the higher energy singlet excited states from which the fluorescence is produced. This energy, thus, becomes lost to decay processes that heat-up the device rather than emit visible light.
Typically, phosphorescent emission from organic molecules is less common than fluorescent emission. However, phosphorescence can be observed from organic molecules under an appropriate set of conditions. Organic molecules coordinated to lanthanide elements often emit from excited states localized on the lanthanide metal. Such radiative emission is not from a triplet excited state. Furthermore, such emission has not been shown to be capable of producing efficiencies high enough to be of practical value in anticipated OLED applications. The europium diketonate complexes illustrate one group of these types of species.
Organic phosphorescence may be observed in molecules containing heteroatoms with unshared pairs of electrons but, typically, only at very low temperatures. Benzophenone and 2,2′-bipyridine are such molecules. Phosphorescence can be enhanced over fluorescence at room temperature by confining, preferably through bonding, the organic molecule in close proximity to an atom of high atomic number. This phenomenon, called the heavy atom effect, is created by a mechanism known as spin-orbit coupling. A related phosphorescent transition is a metal-to-ligand charge transfer (MLCT) that is observed in molecules such as tris(2-phenylpyridine)iridium(III).
The realization of highly efficient blue, green and red electrophosphorescence is a requirement for fall color display applications with low power consumption. Recently, high-efficiency green and red organic electrophosphorescent devices have been demonstrated which harvest both singlet and triplet excitons, leading to internal quantum efficiencies (ηint) approaching 100%. See Baldo, M. A., O'Brien, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E., and Forrest, S. R., Nature (London), 395, 151-154 (1998); Baldo, M. A., Lamansky, S., Burrows, P. E., Thompson, M. E., and Forrest, S. R., Appl. Phys. Lett., 75, 4-6 (1999); Adachi, C., Baldo, M. A., and Forrest, S. R., App. Phys. Lett., 77, 904-906, (2000); Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001); and Adachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863 (2001). Using a green phosphorescent material, fac tris(2-phenylpyridine)iridium (Ir(ppy)3), in particular, an external quantum efficiency (ηext) of (17.6±0.5)% corresponding to an internal quantum efficiency of >85%, was realized using a wide energy gap host material, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ). See Adachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863 (2001). More recently, high-efficiency (ηext=(7.0±0.5)%) red electrophosphorescence was demonstrated employing bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N, C3) iridium (acetylacetonate) [Btp2Ir(acac)]. See Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App. Phys. Lett., 78, 1622-1624 (2001).
In each of these latter cases, high efficiencies are obtained by energy transfer from both the host singlet and triplet states to the phosphor triplet, or via direct trapping of charge on the phosphorescent material, thereby harvesting up to 100% of the excited states. This is a significant improvement over what can be expected using fluorescence in either small molecule or polymer organic light emitting devices (OLEDs). See Baldo, M. A., O'Brien, D. F., Thompson, M. E., and Forrest, S. R., Phys. Rev., B 60, 14422-14428 (1999); Friend, R. H., Gymer, R. W., Holmes, A. B., Burroughes, J. H., Marks, R. N., Taliani, C., Bradley, D. D. C., Dos Santos, D. A., Bredas, J. L., Logdlund, M., Salaneck, W. R., Nature (London), 397, 121-128 (1999); and Cao, Y, Parker, I. D., Yu, G., Zhang, C., and Heeger, A. J., Nature (London), 397, 414-417 (1999). In either case, these transfers entail a resonant, exothermic process. As the triplet energy of the phosphorescent material increases, it becomes less likely to find an appropriate host with a suitably high energy triplet state. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000). The very large excitonic energies required of the host also suggest that the host material may not have appropriate energy level alignments with other materials used in an OLED structure, hence resulting in a further reduction in efficiency. To eliminate this competition between the conductive and energy transfer properties of the host, a route to efficient blue electrophosphorescence may involve the endothermic energy transfer from a near resonant excited state of the host to the higher triplet energy of the phosphorescent material. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000); Ford, W. E., Rodgers, M. A. J., J. Phys. Chem., 96, 2917-2920 (1992); and Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun., 735-736 (1999). Provided that the energy required in the transfer is not significantly greater than the thermal energy, this process may be very efficient.
The quality of white illumination sources can be fully described by a simple set of parameters. The color of the light source is given by its CIE chromaticity coordinates x and y. The CIE coordinates are typically represented on a two dimensional plot. Monochromatic colors fall on the perimeter of the horseshoe shaped curve starting with blue in the lower left, running through the colors of the spectrum in a clockwise direction to red in the lower right. The CIE coordinates of a light source of given energy and spectral shape will fall within the area of the curve. Summing light at all wavelengths uniformly gives the white or neutral point, found at the center of the diagram (CIE x,y-coordinates, 0.33, 0.33). Mixing light from two or more sources gives light whose color is represented by the intensity weighted average of the CIE coordinates of the independent sources. Thus, mixing light from two or more sources can be used to generate white light. While the two component and three component white light sources will appear identical to an observer (CIE x,y-coordinates, 0.32,0.32), they will not be equivalent illumination sources. When considering the use of these white light sources for illumination, the CIE color rendering index (CRI) needs to be considered in addition to the CIE coordinates of the source. The CRI gives an indication of how well the light source will render colors of objects it illuminates. A perfect match of a given source to the standard illuminant gives a CRI of 100. Though a CRI value of at least 70 may be acceptable for certain applications, a preferred white light source will have a CRI of about 80 or higher.
The most successful approaches used to generate white OLEDs described previously involve separating three different emitters (luminescent dopants) into individual layers. Three emissive centers are needed to achieve good color rendering index (CRI) values, as the lines are typically not broad enough to cover the entire visible spectrum with fewer than three emitters. One approach to the design of WOLEDs involves segregating the individual dopants into separate layers. The emissive zone in such a device is thus composed of distinct emissive layers. Kido, J. et. al. Science, 267, 1332-1334 (1995). The design of such a device can be complicated, since careful control of the thickness and composition of each layer is critical for achieving good color balance. The separation of emitters into individual layers is essential to prevent energy transfer between the red, green and blue emitters. The problem is that the highest energy emitter (blue) will efficiently transfer its exciton to the green and red emitters. The efficiency of this energy transfer process is described by the Forster energy transfer equations. If the blue emitter has good spectral overlap with the absorption spectra of the green or red emitters, and the oscillator strengths are high for all of the transitions, the energy transfer process will be efficient. These energy transfers can occur over distances of 30 Å or more. Likewise the green emitter will readily transfer its exciton to the red emitter. The end result is that the red emitter dominates the spectral composition if the three are doped into the film at equal concentrations. With fluorescent dyes the exciton migration lengths are comparatively short and the balance between the three emission colors can be controlled by varying the dopant ratios (more blue is needed than green and more green than red to achieve equal intensity at all three colors). If the dopant concentration is kept low the average distance between dopants can be kept below the Forster energy transfer distance and the affects of energy transfer can be minimized. Having all three dyes within a single layer involves a four component film, with each dopant present at <1%. The preparation of such a film is difficult to carry-out reliably. Any shift in dopant ratio will severely affect the color quality of the device.
The situation with phosphorescent emitters is somewhat different. While the Forster radii of phosphorescent dopants may be lower than for fluorescent dopants, the exciton diffusion lengths can be >1000 Å. In order to achieve high efficiency with electrophosphorescent devices, the phosphorescent materials generally need to be present at much higher concentrations than for fluorescent dopants (typically >6%). The end result is that mixing the phosphorescent materials together in a single layer leads to a severe energy transfer problem, just as observed for fluorescent emitters. The approach that has been used successfully segregates the phosphorescent materials into separate layers, eliminating the energy transfer problem.